Recently, concerns about the environment have increased and a great amount of attention is being paid to the prevention of evaporation of organic solvents and a liquid fuel such as gasoline. Fuel evaporation from a vehicle being parked is roughly divided depending on its production source into evaporation from a fuel tank system and evaporation from an intake system. Hereinafter, a fuel evaporative emission preventing device to prevent the release of fuel evaporative emission from a fuel tank system is referred to as a canister, and an element to be used for a fuel evaporative emission preventing device for fuel vapor from an intake system is referred to as an engine air intake element.
Normally, fuel vapor to evaporate from a fuel tank system is adsorbed and collected by an adsorbent housed in the canister. The canister was developed as a fuel evaporative emission preventing device to prevent the release of the liquid fuel vapor represented by gasoline generated from the fuel tank system, and so far many canisters have been developed and loaded and used in vehicles. As a fuel evaporation preventive material to be used in the canister, activated carbon is used in many canisters, and in the canisters, vapor evaporating from the liquid fuel such as gasoline from the fuel tank is adsorbed by activated carbon, and the adsorbed gasoline vapor is desorbed by the air suctioned when an engine is driven, and the desorbed gasoline vapor is guided into an engine intake manifold (hereinafter, abbreviated to intake manifold) and burned by the engine.
Conventionally, to improve the performance of the canister, development of adsorbents excellent in adsorption and desorption of liquid fuel vapor and development of systems were made. As adsorbents, activated carbon such as coconut shell charcoal, wood coal, and coal, etc., and molded charcoal obtained by molding activated carbon were normally used, and for example, molded charcoal obtained by adding short fibers into granular activated carbon and molding it by using an emulsion as a binder is known (Patent Document 1). Modified activated carbon that was oxidized is also known, and it is described that this is particularly effective for a mixed vapor of gasoline and alcohol (Patent Document 2). Furthermore, use of two kinds of activated carbons different in average filling density from each other for a fuel vapor collecting system has also been proposed (Patent Document 3).
Patent Document 1: Japanese Published Examined Patent Application No. S48-7194
Patent Document 2: Japanese Published Examined Patent Application No. H01-52324
Patent Document 3: Japanese Published Examined Utility Model Application No. H05-17411
As an example of an adsorbent developed by focusing on the pore size distribution of activated carbon, a fuel evaporation preventive material made of an activated carbon fiber having a specific pore size distribution has been disclosed (Patent Document 4). In addition, a method for producing an activated carbon that improves the pore size distribution by adjusting the oxygen concentration during heat treatment has been disclosed (Patent Document 5), and as an example developed by focusing on the hardness, an activated carbon having hardness without greatly reducing the pore size has been disclosed (Patent Document 6). Furthermore, an activated carbon regulated by specific adsorption indicated by the butane working capacity and wear rate has been disclosed (Patent Document 7), and a molded activated carbon regulated by a crushing strength and a specific heat has been disclosed (Patent Document 8).
Patent Document 4: Japanese Published Examined Patent Application No. S61-55611
Patent Document 5: Japanese Published Unexamined Patent Application No. H06-127912
Patent Document 6: Japanese Published Unexamined Patent Application No. H04-190846
Patent Document 7: Japanese Published Unexamined Patent Application No. 2000-313611
Patent Document 8: Japanese Published Unexamined Patent Application No. 2001-322872
On the other hand, as an example developed in terms of the system, for example, canisters having an adsorbent formed by laminating a plurality of activated carbon sintered sheets so as to form spaces therein have been disclosed (Patent Documents 9 and 10).
Patent Document 9: Japanese Published Unexamined Patent Application No. H04-265461
Patent Document 10: Japanese Published Unexamined Patent Application No. H07-269421
When a vehicle is parked for a long period of time, gasoline adsorbed by the adsorbent in the canister is emitted to the atmosphere and pollutes the environment, and in particular, in the USA, evaporation from vehicle fuel is strictly regulated. All over the USA, from 2004, vehicle fuel evaporation control regulations called Tier 2, and in California, called LEV II (Low Emission Vehicle II) are due to be enforced, and according to these new regulations, it is required that the evaporative emission of the gasoline while parking for 72 hours, that is, Diurnal Breathing Loss (DBL) is reduced to a value remarkably lower than conventionally.
Furthermore, the p-ZEV (Partial Zero Emission Vehicle) regulation requires a canister to satisfy an emission value of 5 through 15 mg in general, and to realize this value, conventional activated carbons for canisters are not sufficient, and a canister with high performance has been demanded.
However, the canisters described above were developed in view of adsorption and desorption of gasoline vapor in principal, and were not developed by considering the evaporative emission of the gasoline from a vehicle being parked. Adsorption of fuel vapor is realized by only increasing the amount of activated carbon, however, this increases the pressure drop, resulting in lengthening of the fuel feeding period. The inventors developed as a fuel evaporative emission preventing device which is applicable for such objects, a fuel vapor evaporative emission preventing device for a vehicle including a second canister made of an activated carbon honeycomb by extrusion-molding connected after a canister using a granular activated carbon, and applied priorly for a patent (Patent Document 11).
Patent Document 11: Japanese Published Unexamined Patent Application No. H10-37812
In addition, recently, a canister including a combination of an adsorbent with adsorption capacity more than 35 g/L (liter) of n-butane in the n-butane concentration range of 5 percent by volume through 50 percent by volume at 25° C., and an adsorbent with adsorption capacity of 35 g/L or less of n-butane (Patent Document 12).
Patent Document 12: U.S. Pat. No. 6,540,815
The canisters disclosed in Patent Documents 11 and 12 are introduced as being capable of restraining the leak amount of the evaporative emission of the gasoline even when a vehicle is parked for a long period of time by connecting the second canister after the canister using granular activated carbon, and in these patent documents, use of honeycomb-shaped activated carbon as a second canister has been disclosed. However, the honeycomb molding described in Patent Document 11 is produced by extrusion-molding, and this is easily broken. Use of a honeycomb is also described in Patent Document 12, however, it is made of ceramic and easily broken although its pressure drop is low. In recent years, strength against vibrations has been strongly demanded in view of loading in a vehicle, and the problem in strength of the conventional activated carbon honeycombs has been pointed out.
On the other hand, even when fuel vapor from an intake system is adsorbed, the adsorption method using activated carbon as an adsorbent is effective, and an adsorbent is set and used in the middle of the intake manifold or near an air filter as a combustion air inlet. A large amount of air passes through the intake manifold at a high speed, and therefore, in the case of development of a fuel evaporative emission preventing device in such an intake system, as an engine air intake element to be used (hereinafter, abbreviated to air intake element), the lower pressure drop than that of the canister is required.
As the air intake element, for example, use of a monolith with a honeycomb structure obtained by, for example, mixing activated carbon and a ceramic substance and extrusion-molding and calcining these as an adsorptive filter for removing volatile organic compounds from the air intake system of a vehicle engine has been disclosed (Patent Document 13), and use of a phenol resin and cellulose fibers, etc., which have been mixed, extruded into a honeycomb, and then carbonized and activated, for removal of volatile organic compounds, has been disclosed (Patent Document 14). However, these honeycomb moldings are easily broken although their pressure drop is low, so that the problem in strength against vibrations has also been pointed out as in the case of the canisters in view of loading in a vehicle.
Patent Document 13: U.S. Pat. No. 5,914,294
Patent Document 14: U.S. Pat. No. 5,820,967